Tardigrades survive extreme dehydration by turning into glass

Just when we thought we’d seen all the tricks waterbears keep up
their tiny sleeves, they’re back to surprise us again. Scientists have
revealed one of the tardigrade’s best-kept secrets: how they protect
themselves from harm under extreme dehydration. They do it by turning
into glass. Now we know how.
The trouble with falling from heights isn’t the fall itself; it’s the
sudden stop at the end. In much the same way, the trick isn’t
necessarily that the tardigrade can turn into glass, but that it can
come back from that state and be fine. To accomplish that remarkable
feat, the animals rely on bespoke proteins called tardigrade-specific intrinsically disordered proteins (TDPs).
Proteins have a three-dimensional structure in space, and it’s often
different depending on what the protein is “in” — whether it be water,
methane, carbon dioxide, or a vacuum. TDPs are no exception. In water,
many proteins become hydrated, and they sort of loosen up and act like
kelp, with waving, frondlike arms instead of a tightly folded structure.
But when they start to dry out, TDPs do something special. They form a
kind of bioglass, keeping the waterbears’ innards from harm.
“When the animal completely desiccates, the TDPs vitrify, turning the
cytoplasmic fluid of cells into glass,” said lead author Thomas
Boothby, of UNC Chapel Hill. “We think this glassy mixture is trapping
[other] desiccation-sensitive proteins and other biological molecules
and locking them in place, physically preventing them from unfolding,
breaking apart or aggregating together.”
This forced me to re-evaluate my entire understanding of the notion of glass, much like this recent story changed my understanding of wetting — which has more to do with the tight contact between two things than it does with water.

A glass is an amorphous solid that doesn’t have a
crystalline structure. Even some things that have a crystalline form can
also have a glass form. For example, when you heat and cool some
crystalline sugars, they go through a noncrystalline glass phase. That’s
how hard candies stay crisp and transparent, instead of melting into a
pile of goo: sugar glassing.

In a culinary setting, sugar glassing requires temperatures
in excess of 300 F, but in the tardigrade these TDPs seem to grab onto
sugar molecules and glass them at much lower temperatures. That’s
another cool power of proteins: being able to conduct chemical reactions
far from equilibrium. Being a glass requires “long-range atomic
disorder,” and the mishmosh of proteins and sugars seems to supply that
requirement for chaos.

Freezing and bioglassing both have something in common with
drying, in that when you dry something out, you’re pulling all the water
molecules out of solution and sending them into the ambient air. When
water freezes, what’s actually happening is that it’s becoming crystals;
that has the effect of pulling the water molecules out of aqueous
solution and depositing them into the crystal matrix. That’s why
freeze-drying is a food preservation tactic.

The formation of water ice crystals is why frostbite is so
damaging, and it’s also why frozen fruits and veggies are sort of mushy
and leaky when they thaw. Those needlelike crystals puncture individual
cells and let the cytoplasm leak out. But tardigrades can survive being freeze-dried in the vacuum of space,
and now we’re starting to understand why. Doubtless these continuing
breakthroughs will help us to tie together concepts and plunder the tardigrade’s survival skillset for our own gain.